The importance of particle density in passive pulsing air classifiers is examined. It had heretofore been assumed that passive pulsing classifiers separate particles according to density, although the assumption had never been tested. Theoretically, pulsing isolates the segment of the particle velocity curve where a denser particle falls faster than a less dense particle, before particles achieve terminal velocity. Within this velocity range, particles can be separated according to density. The existence of pulsing in the classifiers, or lack thereof, is verified by flow observations and velocity profiles. Uniform aluminum and plastic plates differing only in density are separated in passive pulsing and nonpulsing classifiers. Passive pulsing configurations separated the particles more effectively than nonpulsing. These results suggest that passive pulsing air classification separates according to a different criterion than nonpulsing, and that particle density is the parameter. In studies with real particle separations, passive pulsing classifiers also outperformed nonpulsing.